656 S Indies in ike Theory of Descent. 



nature of each living form. He errs, however, 

 both in making absolute illimitability a necessary 

 postulate of the theory of selection, as also in 

 inferring the existence of a directive principle 

 from that limitation of variability which is cer- 

 tainly present. " Tendencies to variation " do 

 however exist, not in the sense of a directive 

 power, but as expressions of the different physical 

 constitutions of species, which necessarily cause 

 unequal reactions to the same external actions, as 

 will be more clearly proved below. 4 



This is, of course, a modification of Darwin's 

 original assumption of an unbounded variability 

 not limited in direction ; but Darwin himself has 

 later coincided in the view that the quality of 

 the variations is essentially determined by the 

 nature of the organism. 5 



4 [Eng. ed. This idea, formerly expressed by me, occurs 

 also in Lange (" Geschichte des Materialismus," ii. 265), 

 and is there exemplified in a very beautiful manner by illustra- 

 tions from modern chemistry. Lange compares what I have 

 termed above the " physical constitution" of the organism to 

 the chemical constitution of one of those organic acids- which 

 by substitution of single elements may become transformed 

 into more complicated acids, but which, as it were, always 

 undergo " further development " in only one determined and 

 narrowly restricted course. Here, as with the organism, the 

 number of possible variations is very great, but is nevertheless 

 limited, since " what can or cannot arise is determined before- 

 hand by certain hypothetical properties of the molecule."] 



8 "Origin of Species." 4th German ed., p. 19 ; 5th English 

 ed., p. 6. 



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